I got my bearded dragon two months ago and he is about 11 months old and doing just fine. But for as long as I've had him he's always seemed to have his to left arm slightly yellow and thicker. I'm pretty sure it's just really bad reatained shed, because it looks like it's kind of peeling, and he's had spots like this before that came off, but yellow spots on a beardie never mean well.
I really don't know much about yellow fungus I've never had any dealings with it,so hopefully someone else will chime in.
However, i would definitely take him to vet asap! and keep him away from other reptiles until you know for sure. If it is yellow fungus it's highly contagious. It can also spread to other parts of the body including internal organs. while the picture you have posted does resemble other pictures I have seen of the illness, a vet will be able to provide a proper dignoses
It doesn't look like yellow fungus to me. How long has it looked like that?
Have you tried bathing him to see if that helps with any possible retained shed?
You can try using shea butter or aloe vera to moisturize the skin to see if that works.